fix(storybook): replace HCL code snippets with XML syntax in styling page

[geoffjay]

Closes #54

Summary

• Changed all 6 styling tab labels from ["Preview","HCL"] to ["Preview","XML"]
• Changed panel IDs from _hcl to _xml suffix
• Changed all code editor language="hcl" to language="xml"
• Rewrote all 6 code snippet default-value attributes from HCL block syntax to XML attribute syntax

Part of epic #9.

[geoffjay]

This change is part of the following stack:

• fix(storybook): replace HCL code snippets with XML syntax in styling page #71 ◀

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